Ferrite antenna coupled to radio frequency currents in vehicle body

ABSTRACT

An antenna system in which a Faraday cage acts as the primary antenna which intercepts the electromagnetic waves and reradiates them to a secondary antenna located within the Faraday cage.

United States Patent [191 Volkers, deceased et a1.

FERRITE ANTENNA COUPLED TO RADIO FREQUENCY CURRENTS IN VEHICLE BODY In e ntorsz Walter R. Vol kers, deceased, late of Sand Point, by Daphne Volkers, executrix; Edward N. Willie, Locust Valley, both of N.Y.

Continuation of Ser. No. 658,995, Aug. 4, 1967, abandoned.

U.S. Cl .343/712, 343/788 Int. Cl. ....H0lq 1/32 Field of Search ..343/711, 712, 720

[111 3,717,876 51 Feb. 20, 1973 References Cited UNITED STATES PATENTS 9/1950 Williams et al ..343/712 9/1950 Williams et al ..343/712 7/1926 Albro ..343/720 9/1949 Clough ..343/711 7/1963 Piccinini ..343/71 1 Primary Examiner-Eli Lieberman Attorney-Darby & Darby ABSTRACT An antenna system in which a Faraday cage acts as the primary antenna which intercepts the electromagnetic waves and reradiates them to a secondary antenna located within the Faraday cage.

1 Claim, 18 Drawing Figures CONTOUROF WIND 1 SCREEN) 3 PATEMED Q 3.717, 876

SHEET 10F 8 FIQI.

I ALL L CURRENT Y b V ?E%E) t I DE'\RY I23 H-FIELD Y y AND FLUX V 9% INSIDE 3 SECONDARY r H-FIELD v 5 AND FLUX 4 r A K V I V Flqs.

Sh'EET u 07 a PATENTED FEBZ 0 I973 PATENTEB r5820 ma sum 70F 8, v

PLANE OF COIL IS AT RIGHT ANG TO CENTRE LINE OF MAIN CYLINDER PAIENTEDFEHZOISB f E E 3,717,878.

, SHEET 8 OF {3 333 I 289 TQTRANSMITTER 288 E Eek RECEIVER FERIIITE ANTENNA COUPLED TO RADIO FREQUENCY CURRENTS IN VEHICLE BODY This application is a continuation of application Ser. No. 658,995, filed Aug. 4, I967 and now abandoned.

A Faraday cage is a three dimensional, field-disturbing conductor which may be, but need not be, a complete enclosure. It was an early notion of wireless communications engineering that the inside of an electrically conductive enclosure or Faraday cage would be inherently free of radio fields. The present invention recognizes that this early notion was in error. More particularly, the present invention recognizes that under certain conditions, a Faraday cage may not only fail to keep out the electrostatic and magnetic fields of a passing electromagnetic wave but can even magnify or concentrate them.

It is therefore an object of this invention to provide an efficient, cascade antenna system in which a Faraday cage serves as a primary antenna and a reradiator which can be linked with a secondary antenna for connection to a conventional tran'smitter or receiver. Such antenna systems have considerable practical value whenever certain mechanical or electrical considerations make conventional antenna designs impractical or undesirable.

The present invention makes use of the fact that a Faraday cage which differs in its linear dimension from a quarter wave length, or a multiple thereof, creates a disturbance of the electrostatic and magnetic fields of a passing electromagnetic wave through an enforced redistribution of equipotential field lines, or planes. This partial E and H-field collapse" (or boost) is accompanied by substantial equalizing currents, flowing in the conductive enclosure of the Faraday cage. Such currents then create strong secondary magnetic fields which, through inductive conversion, can be translated into signal voltages, signal power and signal currents for receiving purposes. This process can then be reversed if transmission is desired.

In accordance with the preferred form of this invention a sub-quarter-wave length Faraday cage is used to convert the E or H-tields of a passing wave into a secondary H-field which is then detected by a magnetic antenna. This cascading two antennas, one of which acts as a reradiator, can also be described as an indirect antenna operation, in which the Faraday cage can be labeled as the primary antenna," and the magnetic antenna within its enclosure as the secondary antenna. The effectiveness of such a system depends, among other factors, upon its electromagnetic conspicuousness, i.e. E or H-field disturbance which the Faraday cage creates.

One advantage of a Faraday cage antenna is its natural physical sturdiness which is due to its three-dimen-- sional structure which is in contrast to the linear structure of conventional antennas, such as monopoles and dipoles. The Faraday cage antenna is therefore less lia bleto be damaged by physical environmental factors. Further, there is frequently the possibility of using an existing mechanical structure, such as the steel beams of a building, the body of a motor car, or a section of an aircraft frame as the primary Faraday cage antenna. By placing a suitable secondary antenna near a convenient part of the existing Faraday cage, the antenna system can be completed.

In airborne applications the present Faraday cage antenna offers the additional advantage of being aerodynamically inconspicuous while being elec tromagnetically conspicuous. It avoids the extra air resistance of conventional aircraft antennas which reduce speed and increase fuel consumption andare therefore generally considered an unwelcome but necessary accessory.

These and other objects and advantages of the present invention will be apparent to those skilled in the art from the following detailed description and accompanying drawings which set forth the principle of the present invention and, by way of example, the best modes which have been contemplated for applying that principle.

In the drawings:

FIG. I is a perspective view of a Faraday cage of the screen-room type.

FIG. 2 is a plan view of a Faraday cage antenna system including a secondary antenna.

FIG. 3 is a perspective view of a Faraday cage antenna system in which two sides of the Faraday cage are open.

FIG. 4 is a perspective view of a Faraday cage antenna system in which two conductive sides are connected by conductive corner posts.

FIGS. 5 and 6 are plan views-of Faraday cage antenna systems having corner posts designed to increase the coupling efficiency between the primary and secondary antennas.

FIGS. 7 and 8 are detailed views of corner posts in side elevation partly broken away and in cross-section.

FIG. 9 is a side elevational view of a Faraday cage antenna system in which the body of an automobile is used as the Faraday cage.

FIG. 10 shows the redistribution of equipotential lines aboutthe body of an automobile.

FIG. 11 is a perspective view of a Faraday cage antenna system according to the present invention in which the Faraday cage is in the form of a shortened parallel plate condenser.

FIG. 12 shows the redistribution of equipotential lines caused by the antenna system of FIG l I.

FiG. 13 is a perspective view of a Faraday cage antenna system incorporating a transformer having a large air core.

FIGS. 14 and 15 are cross-sectional views of aerospace vehicles including Faraday cage antenna systems according to the present invention.

FIG. 16 is a perspective view of a section of an aerospace vehicle including a Faraday cage antenna system of the present invention.

FIG. '17 is a perspective view of a section of an aerospace vehicle including a Faraday cage antenna system the directionality of which may be changed by electrical means.

FIG. 18 is a perspective view of a modified form of Faraday cage antenna system according to the present invention.

The rectangular Faraday cage shown in FIG. 1 is a typical shielded enclosure, or screen room, such as is commonly used in electronic laboratories for the purpose of keeping out unwanted signals. The cage 1, shown in FIG. 1, does not contain a second, inner cage such as is commonly used when more effective shielding is required. The need for such a second cage is usually explained as being due to the meshed wire structure 2 of the walls 3-6, floor 7 and ceiling 8 of the outer cage 1 which fail to prevent electromagnetic waves from reaching through the small spaces between the wires. But this simple explanation cannot satisfactorily explain the following two observations familiar to those who have designed screen rooms. (1) Significant tightening of the wire mesh (increasing the copper/space ratio) yields an insignificant improvement of shielding-effectiveness, and (2), a second inner cage increases the effectiveness of signal rejection by a factor which is much greater than can be attributed to the increase of the copper/space ratio, due to the addition of the second cage.

Further, it has been observed by screen room designers that (3) a single screen room which is only moderately efficient in the first place, becomes less efficient as frequency is lowered towards the LF and VLF region, and (4) also becomes less efficient as frequency is raised towards the VHF and UHF region.

The present invention recognizes that the above observations are related to the height 9 of the screen room relative to a quarter wave-length of the signal which is to be rejected, assuming that the signal is vertically polarized as signified by arrow 10, in FIG. 1. Stated in another way the present invention recognizes that the above observations are related to whether the structure is electromagnetically conspicuous or electromagnetically inconspicuous.

Whether an antenna is electromagnetically conspicuous or inconspicuous is determined by its influence upon the surrounding electrostatic and magnetic fields of an electromagnetic wave. More specifically, whether or not the antenna influences these fields may be determined empirically by comparing the fields before and after insertion of the antenna. Electromagnetically, the least conspicuous, and therefore most inconspicuous antenna is a thin, vertical, grounded, linear quarter-wave antenna, from which no energy is extracted and which stands alone in a vertically polarized far-field of an electromagnetic wave in perfect E/H energy-balance. Such an antenna is capacitively linked with the E-field, through its radiation capacity, and inductively linked with the H-field, through its radiation inductance. Both fields induce voltages in it in the same manner in which the electromagnetic wave itself induces voltages in adjacent, incremental sections of space, causing the electromagnetic wave to act as a travelling resonance in space. Mutual inductance and mutual capacity between incremental sections of space, due to its permeability (unity), and mutual capacity, due to its dielectric constant (unity), are the transmission links which make it possible for a travelling resonance, i.e. an electromagnetic wave, to propagate itself through space,

The above-described inconspicuous, self-resonant monopole antenna differs so little in its electrostatic and magnetic characteristics from space itself that it hardly disturbs normal wave propagation in its vicinity. Just as the E and H fields of the passing electromagnetic wave are in perfect balance, so are radiation capacity and radiation inductance of the antenna, causing it to appear to be non-reactive from a point in its cross-section near ground. From this point, the antenna appears as a resistor, and the commonly used term radiation resistance" takes this into account.

Radiation resistance differs in two important aspects from radiation capacity and radiation inductance: (1) it is not a real resistance, while the other two parameters are real. (2) It is a boundary effect, which the other two parameters are not.

Radiation resistance is not real, but is just the equivalent load which space presents to the antenna, as the antenna forces space to change from its previously unexcited state into the excited state. This transition from one state to the other is accompanied by a conversion of energy from one form into another, viz. from real energy into potential energy. The energy passing through the antenna lead is real and therefore appears to be associated with a resistance. The energy contained in the travelling electromagnetic wave is potential energy because of the essentially purely reactive properties of space. The commonly adopted term radiation resistance of space" is therefore considered to be somewhat misleading. If space actually has resistive properties, electromagnetic wave propagation would be so severely attenuated that wireless communications would be restricted to very short distances, and the light of distant stars could never reach our planet.

As to the boundary nature of radiation resistance, it is noted that the above-described real-to-potential energy conversion occurs at the antenna, not during wave propagation in space. On the other hand, capacitive and magnetic linkages between an antenna and space are identical in nature with the linkages between adjacent sections of space, particularly in the case of an electromagnetically inconspicuous antenna.

The proper understanding of the concept of radiation resistance plays an important part in the understanding of the Faraday cage and its use as the primary antenna in an indirect antenna system. The Faraday cage is a device which disturbs the originally perfect energy balance between the E and H fields of an otherwise undisturbed electromagnetic wave. Because the radiation resistance of space is not real, it is not an attenuator of wave energy at the location where the energy unbalance between the E and H fields is caused. If no other major attenuators are present( all associated components being essentially reactive), the law of energy conservation will cause the H field to increase in strength, as the E field is decreased, and vice versa. Experiments have confirmed this reasoning. A Faraday cage weakens the E-field, as it is expected to do, but it may strengthen the H-field, which it is not normally expected to do. It is this second effect which makes the present Faraday cage antenna possible.

A Faraday cage weakens the E-field because it is electromagnetically conspicuous, i.e. it significantly disturbs the original E and H fields and their energy balance. This feature of Faraday cages can best be explained by reference to the above-mentioned four observations with regard to the screening efficiency of Faraday cages, and its variations with frequency. All four of the above observations are, as stated, related to the physical height 9, in FIG. 1, of the screen room, relative to a quarter wave length of the signal which is to be rejected.

The screen room 1 in FIG. 1 differs from the idealized, inconspicuous linear antenna in that it is an essentially cubesshaped three-dimensional device. The walls 3-6 of screen room 1 might be thought of as a large number of thin, parallel, vertical, electromagnetically inconspicuous monopole antennas only if the following three conditions were fulfilled: (l) exact quarter wave length height, (2) insulation from each other and (3) insulation from the conductive ceiling 8 of screen room 1.

In fact, each of these three conditions is generally violated. (l if height is less than a quarter wave length, small E-field equalizing currents will flow in the narrow monopoles. However, this effect is relatively small and can, for all practical purposes, be neglected. (2) Failure to insulate the narrow monopoles from each other has a more pronounced effect. Because the sections of the E-field and the H-field with which the narrow monopoles are linked are out of phase due to the waves travelling time as it sweeps past and across the Faraday cage, circulating currents are thus set up which tend to reduce vertical voltage drops. (3) Failure to insulate the ceiling 8 is, however, the most significant. Ceiling 8 acts as a very effective capacitive termination for each of the narrow, parallel monopoles.

' This capacitive termination, due to its large radiation capacity, severely disturbs the otherwise near-perfect reactive balance between the inductive and capacitive properties of each monopole, thus causing heavy equalizing currents to flow within the conductive walls 3-6 of the screen room 1.

The three current producing mechanisms described above partially explain the substantial currents flowing in the walls of the screen room 1 of FIG. 1. Additional mechanisms, generally too complex for the purposes of this discussion, also participate in the generation of wall currents in a subquarter-wave Faraday cage. One such additional mechanism, namely the E-flux collecting earth-contour-change effect, will be discussed in connection with the description of certain Faraday cage antenna systems. The earth-contour-change effect causes the electromagnetically conspicuous antenna to i look larger, but without changing the relationship between its physical dimensions and quarter wave length. In other words, the earth-contour-change effect does not reduce the antennas desirable conspicuousness, yet causes it to intercept a larger electrostatic flux. Thus the earth-contour-change effect further increases the efficiency of the Faraday cage antenna, as will be described in greater detail hereinafter.

In FIG. 1' the above-mentioned wall currents are represented by a single, vertical arrow 11, in the right wall 5 of the cage, which represents, in lumped form,

the total current distribution in that wall. The wall current 11 induces secondary magnetic fields and magnetic fluxes both inside and outside cage 1 as represented by arrows l2 and 13, respectively. Magnetic fields 12 and 13 either buck orboost the primary H-field 14 of the passing electromagnetic wave, de' pending on its incidental direction of travel. Penetration of the primary I-I-field into the interior the cage 1 is normally insignificant because of the shielding effect created by eddy currents in the walls of the cage.

In addition to the secondary H-tields l2 and 13 inside and outside cage 1, there are also secondary E- fields inside cage 1. These E-fields are generated by both the primary and the secondary H-fields. More accurately, the secondary E-fields are the result of electromotive forces induced within the walls by the primary and secondary I'I-fields. Through capacitive linkage with space near the walls these electromotive forces induce the above-mentioned secondary E-fields which are not shown in FIG. 1, in order to keep the illustration simple, also because the secondary E-fields are not directly involved in signal transfer from the primary Faraday cage antenna to the secondary magnetic antenna, in accordance with the present invention.

Referring once again to the four observations familiar screen room designers, the following explanations are offered:

I. A tightening of the wire mesh fails to improve the effectiveness of shielding obtainable with a single con ductive enclosure, because secondary fields are generated inside the enclosure. These secondary fields are not to be mistaken for penetration of the primary fields into the Faraday cage through the mesh.

2. A second, inner shield is highly effective because it is exposed only to the weak, secondary fields of the outer shield. As a result, the secondary fields of the inner shield are correspondingly further weakened.

3. The increased leakage of LP and VLF signals through a single Faraday cage is not, in reality, increased leakage, but rather increased secondary fields, resulting from increased wall currents due to increased electromagnetic conspicuousness of the Faraday cage, as its height relative to a quarter wave length, is decreased.

4. The increased leakage of VHF and UHF signals through a single Faraday cage is actually leakage and not the effect of secondary fields. As frequency increases, the Faraday cage becomes more nearly electromagnetically inconspicuous because its height relative to a quarter wave length is increased In the light of the foregoing analysis, an efficient Faraday cage antenna system should have the following properties:

I. The Faraday cage or primary antenna should be electromagnetically conspicuous so'as to maximize the induced E-field equalizing currents and thus create strong secondary H-fields.

II. Unless means are available for coupling of the secondary antenna or array of secondary antennas, all of the equalizing currents flowing in a structurally uniform Faraday cage, the structure of the Faraday cage should be non-uniform so that the equalizing currents will be confined to a relatively small cross-section of the structure in order to generate a strong concentrated magnetic field, so that a smaller, simpler and more efficient secondary antenna can be used.

III. The Faraday cage need not be fully enclosed as long as equalizing currents and the corresponding magnetic fields are generated for detection by the secondary antenna.

FIG. 2 is a plan view of an example of a Faraday cage antenna system which resembles the substantially cubeshaped screen room 1 of FIG. 1 but further includes structural details which increase its electromagnetic conspicuousness, and magnetic field concentration. Faraday cage 11 includes four conductive walls 12-15, which may be solid or of meshed wire, a conductive floor 16, and a conductive ceiling, not shown. Faraday cage 11 is structurally reinforced at its four corners by vertical posts 17-20, which are also made of conductive material. Alternative magnetic secondary antennas 25 and 26 each comprise a ferrite core 21 carrying a coil 22 with terminals 23 and 24. Both alternative secondary antennas 25 and 26 are located near vertical post 17, antenna 25 being inside cage 11 and antenna 26 being outside cage 1 1.

Neglecting, for the time being, the influence of the skin effect upon current distribution over the various cross-sectional areas connecting floor 16 and the ceiling of cage 11, it can be seen that if the cross-sections of the four walls 12-15 are sufficiently small in comparison with the cross-sections of the four vertical posts 17-20, most of the ceiling-to-floor E-field equalizing current will flow through the four posts 17-20, and consequently, the magnetic field intensity will be greater near the posts 17-20 than it is, for example, near the centers of the walls 12-15, and, accordingly, the signal voltage developed between the terminals 23 and 24 of coil 22, will therefore be correspondingly greater when the secondary antenna is located near a corner post.

In addition to concentrating the secondary magnetic fields, posts 17-20, by virtue of their relatively large cross-sections, are of minimum inductance and hence maximum current flows through them, thus increasing the electromagnetic conspicuousness of the Faraday cage antenna.

In addition to being electrostatically conspicuous, as explained in connection with FIGS. 1 and 2, a Faraday cage may also be magnetically conspicuousdue to the shorted single-turn loop action of the cage. Magnetic conspicuousness also causes equalizing currents to flow within the walls of the Faraday cage, thus creating its own secondary fields which may be intercepted by a suitable secondary antenna.

FIG. 3 shows a Faraday cage antenna 300 in which two sides of the cage are open. This makes the cage a single-turn, shorted loop of width 305 having a roof section 301, wall sections 302 and 303, and a floor section 304. If the circumference of the loop is significantly smaller than a quarter wave length of the signals intercepted, both the radiation resistance and the radiation inductance of the loop will be small, causing the circulating currents to be heavy. Further a shorted loop having an appreciable width 305 acts as an effective shield against E-fields in spite of the absence of enclosing walls on its right and left. Such a structure is called an open Faraday cage. Assuming vertical polarization of the incident wave, an open Faraday cage keeps the primary E-field out, through the well known shading effect, between its parallel ceiling 301 and floor 304.

As previously described in the general discussion of electromagnetic wave propagation, any essentially lossfree reduction of E-field energy, through insertion of an E-field/H-field energy balance disturbing body, such as the shorted loop of FIG. 3, strengthens the I-I-field in the vicinity which, in turn, increases the current flowing in the loop. Arrow 306 identifies the H-field equalizing currents. A magnetic secondary antenna, such as ferrite bar 307, carrying a winding 308 with terminals 309 and 310, can be used to intercept the magnetic flux 311 induced by the II-field equalizing current if the secondary antenna is placed either near wall 302 (as shown) or near ceiling 301, floor 304 or the opposite wall 303.

Different forms of secondary antenna can be employed. Rather than the small ferrite bar 307 shown in FIG. 3, a larger, wider, longer or thicker ferrite may be used, or more than one ferrite may be used on two, three or four sections of the shorted loop, or the ferrite core, or cores, may be replaced by air cores, and the secondary antenna coils may be connected in series or in parallel, etc.

The Faraday cage antenna 320 shown in FIG. 4 differs structurally from the open, shorted loop-type of Faraday cage antenna 300 shown in FIG. 3 in that it includes provisions for alternative open and enclosed cage operation, depending upon whether there is full coverage, partial coverage or non-coverage of its wall space by conductive material. The structure 320 shown in FIG. 4 may be called a temple-type Faraday cage. It is a Faraday cage which has vertical posts, or pillars, 321-324 connecting its conductive ceiling 325 to its conductive floor 326 and may or may not have conductive walls 327-330 between the four pillars 321-324. If there are no walls in the indicated locations, the structure is an open temple-type Faraday cage. If there are only two conductive walls which are opposite to each other such as, for example 328 and 330, it is a partially open temple-type Faraday cage and if there are four walls, it is a fully enclosed conventional Faraday cage.

The electromagnetic conspicuousness of the templetype Faraday cage differs in accordance with the number of walls provided. For example, when all four walls are present the temple type Faraday cage 320 of FIG. 4 is substantially the equivalent of the cages shown in FIGS. 1 and 2. For the reasons described, it keeps out the primary E-field and it somewhat attenuates the primary I-l-field by means of eddy currents flowing in its walls 327-330 and partially completing their paths through ceiling 325 and floor 326. The full Faraday cage 320 can thus be described as a near perfect E-shield and a somewhat less than perfect H- shield, the degree of shielding depending upon the physical dimensions of the structure relative to a quarter wave length as previously described. If only two conductive walls 328 and 330 are provided between the comer posts 321-324 the Faraday cage 320 shown in FIG. 4 becomes essentially the equivalent of the single turn, shorted loop Faraday cage of FIG. 3, identical E and H field polarization being assumed in both figures. An open Faraday cage essentially retains its capability to keep out the primary E-field from within its boundaries due to the well known shading effect between its parallel ceiling 325 and floor 326. Concerning the H-field, there are two opposing effects which affect its intensity within the boundaries of the cage. (1) Although the partially closed temple-type Faraday cage has only two walls 328 and 330, ceiling 325 and floor 326 all of which are at right angles to the indicated direction of I-I-field polarization and should, therefore, not be subject to the induction of eddy currents, minor eddy currents will nevertheless flow in both the ceiling 325 and floor 326 of the temple cage. These are the result of phase shifts between themagnetically induced electromotive forces due to travel time of the l-I-field as the electromagnetic wave sweeps over theceiling 325 and floor 326 of the cage 320. Lbsses accompanying these small eddy currents tend to slightly reduce H-field intensity inside the cage. (2) On the other hand, the conservation of energy in any sudden E-field/H-field energy balance disturbance transfers a major part of the locally suppressed E-field energy into the I-I-iield, thereby strengthening the l-I-field inside and near the open cage and usually more than offsetting the attenuation due to eddy currents.

With no walls between the four corner posts 321-324, the Faraday cage shown in FIG. 4 is of the fully open-temple type. The primary E-field will fail to penetrate inside the boundaries of the temple, while the l-I-field not only penetrates but may even be re-inforced. The I-I-field interception properties of the fully open temple-type Faraday cage antenna system of FIG. 4 will now be discussed. The functional identity between the shorted, single turn loop of FIG. 3, and the partially open temple-type cage of FIG. 4 has been established. If the two walls 328 and 330 of the partially open cage are removed, the functional identity between the two configurations remains undisturbed. The two front posts 321 and 322 in FIG. 4 correspond to the front vertical section of the loop 303 in FIG. 3, the rear posts 323 and 324 in FIG. 4 correspond to the rear section 302 of the loop in FIG. 3, and ceiling .328 and floor 330 in FIG. 4 correspond to sections 301 and 304 in FIG. 3.

There are three important aspects of the performance of a fully open temple-type Faraday cage antenna as an interceptor of H-fieldsz (1) Because it is in the form of a loop the open Faraday cage antenna is directional. (2) Because it has four posts, it has six possible current loops but these may be treated as a single, equivalent loop. (3) Its magnetic conspicuousness is determined less by the simple dimensional considerations as in the case of E-field interception than by radiation resistance, as will be explained in greater detail hereinafter.

The simple shorted loop Faraday cage antenna of FIG. 3, can be treated as a closed transmission line or delay line in an H-field having an arbitrarily phased e.m.f. generator in each of its vertical sections, the two generators being connected in parallel by two equal sections of the delay line, the length of each section being equal to one-half the circumference of the loop. If the lengths of the delay lines match the phase delay between the generators due to travel time of the intercepted wave from one vertical section to the other, no circulating or equalizing currents will flow between the generators. Only standing wave currents will flow in each delay line section, and the two sections might be divided at their midpoints into two self-resonant and, with reference to current and voltage, completely identical and parallel monopole antennas with no circulating or equalizing current exchange between them.

To establish the ideal phasing to minimize equalizing currents and thus minimize the magnetic con- .spicuousness of the loop, the loop must be relatively large. Its vertical conductors must be spaced a half wave length apart and its circumference must be an odd multiple (3,5,7, etc.) of a half wave length. Since the radiation resistance of the single loop Faraday cage antenna (in excess of 1,000 ohms) and since the radiation resistance is electrically in series with the radiation inductance, magnetic conspicuousness cannot be obtained simply by moderate reductions of the loop circumference. Even if the circumference is reduced to a single half wave length and the spacing between the vertical sections is substantially below a half wave length, radiation resistance will be over ohms thus considerably limiting the equalizing currents and accordingly reducing magnetic conspicuousness. As a practical rule it has been found that the actual or equivalent loop circumference should not appreciably exceed a half wave length and can, in fact, be made considerably smaller.

In evaluating the three principal configurations of the temple-type Faraday cage of FIG. 4 as a primary antenna, consideration must be given to the linkages of the primary antenna with the E-field and with the H- field. Both fields are energy sources. Each carries its own flux which can be intercepted by areas, or crosssections, which the Faraday cage antenna presents to it. Thus, the temple-type Faraday cage of FIG. 4 can be regarded as either a magnetic primary antenna or an electrostatic primary antenna, or as a combination of both.

The discussion of Faraday cages, in connection with FIGS. 1 and 2, laid down certain performance principles and design rules for a fully enclosed Faraday cage antenna which is linked principally with the E-field and E-flux of a passing electromagnetic wave. In particular, it was pointed out that a concentration of the E-field equalizing currents flowing in structural parts or areas of the Faraday cage near where the secondary magnetic antenna should be installed, will increase the efficiency of the antenna system. The same considerations apply to a Faraday cage antenna which is principally linked with the H-field and H-flux of the passing electromagnetic wave. The H-field equalizing currents should preferably be concentrated in those structural parts or areas of the Faraday cage where the secondary magnetic antenna system is to be located. Such current concentration will improve the efficiency of Faraday cage antenna system as described above.

The temple-type Faraday cage antenna of FIG. 4 provides both electrostatic and magnetic field linkage with space. Electrostatically, ceiling 325 and floor 326 can be considered as the two electrodes of a parallel plate condenser antenna which are shorted by the four corner posts 321-324 and which intercept the vertically polarized E-flux. Magnetically, the Faraday cage antenna of FIG. 4 presents itself to the horizontally polarized H-field as a shorted, single turn loop.

As stated previously, the wall-less Faraday cage of FIG. 4 effectively shields the enclosure bounded by ceiling 325 and floor 326 and the planes connected their edges from the primary E-field. The E-field shielding capability of the open temple-type Faraday cage of FIG. 4 therefore corresponds to that of the fully enclosed Faraday cage of FIG. 2. The total E-field equalizing current flowing between the ceiling and floor in each instance is essentially the same, assuming that certain of their physical dimensions such as the cross-sections, the surface areas and the conductivities of their four corner posts are equal. A minor differences between the equalizing currents in the two cases can be expected because the Faraday cage of FiG. 2 has conductive walls, whereas the Faraday cage of FIG. 4 has no conductive walls. This makes its ceiling-to-floor shorting impedance of the cage of FIG. 4 slightly larger than that of the cage of FIG. 2. On the other hand, the absence of walls in FIG. 4 forces the entire ceiling-to-floor E-field equalizing current to flow through the four corner posts 321-324, including post 323, near to which the secondary ferrite antenna 331 is located. Whether or not the diversion of equalizing current from the posts 1720, by the conductive wall 12-15 in FIG. 2, causes a greater reduction of the current which flows in the corner posts than that caused by the moderate increase of shorting impedance between ceiling 325 and floor 326 due to the absence of walls and the resulting reduction of electrostatic conspicuousness of the entire cage in FIG. 4, will not be dealt with here.

Of great importance however, is the problem of wasteful sharing of the equalizing currents between the four posts 321-324 in FIG. 4. The efficiency of the present Faraday cage antenna system depends on the unwasted portion of the total equalizing current which flows in the cage structure near the secondary antenna, and on the portion of the magnetic flux induced by this unwasted portion of the total equalizing current which is actually linked with the secondary antenna. These two operating parameters of a Faraday cage antenna system are called the current coupling efficiency," and flux coupling efficiency, their product being the total flux transfer efficiency of the antenna system which is an indication of the overall efficiency of conversion of the primary E-flux and/or H-flux, into the secondary magnetic flux which is effectively linked with the windings of the secondary antenna.

For example, it can be seen that the total flux transfer efficiency of the Faraday cage antenna system of FIG. 4 is rather poor. Its current coupling efficiency is about 25 percent, since its single secondary ferrite antenna 331 is linked with only one of the four equalizing current carrying posts 321-324. And assuming that the ferrite, due to its small dimensions and failure to fully encircle post 323, can be expected tointercept not more than percent of the total secondary magnetic flux surrounding the post, the flux coupling efficiency of the system can be assumed to be no better than 5 percent. The total flux transfer efficiency of the system would therefore be no better than 1.25 percent.

The importance of improving the total flux transfer efficiency is therefore apparent, and a variety of techniques for accomplishing this, will be described below. Such techniques are generally applicable to E- field as well as H-field equalizing currents.

The Faraday cage antenna system of FIG. 5 is similar to that shown in FIG. 2, and identical part numbers are used where applicable. The major difference from FIG. 2 is that in FIG. 5, corner post 17 has a substantially enlarged cross-section causing it to carry a larger portion,

of the total equalizing current than the remaining three posts 18-20. Post 17 therefore induces a larger magnetic flux in its vicinity than the other posts 18-20. The triangular cross-section of post 17 further increases the efficiency of the arrangement by permitting closer average proximity of the straight ferrite 21 to the surface of post 17 and thus promoting better flux interception.

The Faraday cage antenna system of FiG. 6 is similar to that shown in FIG. 5 except that the enlarged corner post 17 has a circular cross-section and is fully surrounded by a toroidal coil 21. The toroid has a ringshaped core 27, which may or may not be a ferrite, and winding 22 which ends in terminals 24 and 27.

Another technique for improving the magnetic flux linkage is to provide a plurality of secondary antennas. These can be located near each of the four corner posts of the cage. For example, two secondary antennas would double the flux linkage, three would triple it and four would quadruple it. The windings of the secondary antennas may be connected in series or in parallel.

The examples in FIGS. 5 and 6 show how total flux transfer efficiency of a Faraday cage antenna can be improved by improving current coupling efficiency. Further improvement is possible by also improving flux coupling efficiency. For example, in FIG. 7, post 51 is part of an open or partially open natural Faraday cage, such as, for example, the wind-screen corner post, or the rear window corner post or a door post of a motor car. FIG. 7a is a side view in cross-section of post 51 and FIG. 7b is a cross-section taken along the line b-b of FIG. 7a. The two recessed contours 52 and 53 of the otherwise rectangular profile of post 51 mate with the overlapping parts of the doors or windscreen, not shown. Lines 54 and 55 indicate the roof and parts of the chassis, or bonnet, or boot, to which post 51 is attached. Post 51 may be welded, riveted, brazed or otherwise attached to these structures, or may form an integral part of them.

A section 57 of a single turn winding is located in tubular opening 56 within post 51. Thus embedded in the post 51 where magnetic field strength is negligible, wire 57 acts as a voltage reference point for the electromotive forces which are induced in the two horizontal sections 58 and 59 of the winding and vertical section 60 which is located outside post 51. The ends 61 and 62 of the winding can then be joined to the input or output terminals of a receiver or transmitter or to other windings, not shown.

The above described winding is a long, narrow loopantenna wound around an air core 63, which may be replaced by a ferrite core, if convenient. The loop intercepts a major part of the magnetic flux circulating around post 51 in spite of the fact that it protrudes very little beyond the post. As shown in FIG. 7, horizontal sections 58 and 59 and vertical section 60 of the loop are mechanically held and protected by three tubular enclosures 64, 65 and 66, which may be made of either conductive or non-conductive material as long as at least a portion of one of them is nonconductive so as not to create a shorted secondary winding which would deflect at the magnetic flux from loop 57-60. in accordance with well known transformer principles.

The single turn winding of FIG. 7 may be replaced by a multi-turn winding and may be used either as the only secondary antenna of a system or as one of a plurality of secondary antennas, each of which is associated with its own post. The number of turns and the parallel or series interconnection between coils will be selected to in FIG. 7, post 51 is the door post of a motor car. Other applicable identification numbers are also repeated. Good flux interception over the length of post 51 is achieved by four equally spaced ferrite antennas 71-74, which carry four series-connected coils 75-78, ending in terminals 79 and 80. As shown, the ferrite antennas 71-74 may be fairly widely spaced without creating an appreciable loss of flux coupling efficiency, since each ferrite, because of its high permeability, deflects and collects secondary H-flux lines in its vicinity, thereby making the flux interception almost total.

Experiments have confirmed that it is possible to improve the total flux transfer efficiency of a Faraday cage antenna system by improving current coupling efficiency and flux coupling efficiency. Actually, when the above described techniques for improving flux transfer efficiency are used the Faraday cage antenna system of the present invention can outperform conventional linear antennas, particularly if a relatively large natural Faraday cage, such as a motor car body, is used as the primary antenna. It has also been found that, at relatively low frequencies such as in the broadcast bands, primarily magnetic linkage of the Faraday cage with space yields larger signals than primarily electrostatic linkage. Electrostatic linkage, on the other hand, becomes more effective at higher frequencies.

In order to determine the approximate cross-over point, where the performance of a conventional, essentially electrostatic S-foot whip antenna can be equalled by a Faraday cage antenna system using the body of the motor car as the primary antenna, it was found necessary to deliberately reduce the flux transfer efficiency by using only a single, small ferrite secondary antenna placed near a windscreen post as shown in FIG. 9. This arrangement duplicated sensitivity, bandwidth and signal-to-noise ratio which were normally obtained with a -foot whip antenna in tests covering a frequency range of 0.5 to L6 megacycles. These tests did not taken into consideration the possible signal-attenuation due to the magnetic directionality of the temple-type Faraday cage antenna which might present a problem in a motor car application unless special means were provided for equivalent rotation of the car, aswill be described in greater detail hereinafter.

As shown in FIG. 9, a ferrite stick 31 measuring 2 rs inches long with a inch X 5/32 inch cross-section and a permeability of approximately 25 was mounted on the right front windscreen post 32 of a motor car. It is estimated that this ferrite stick 31 intercepted approximately percent of the total magnetic flux surrounding post 32. In addition, it is estimated that post 32 carried only about l0 percent of the total equalizing currents between roof 56 and the chassis 37, bonnet 38 and boot 39 of the motor car. Therefore, the total flux transfer efficiency of the test system was only on the. order of 1 percent. This clearly indicates the very substantial reserve performance to be found in Faraday cage antennas, particularly if the cage itself is a large natural cage, such as an automobile or aircraft body.

But the surprisingly effective performance of the above-described Faraday cage antenna cannot be fully explained by the relatively large dimensions of the natural Faraday cage alone. An important effect, briefly referred to above, considerably aids the Faraday cage in its function as a primary antenna. This is the earth contour changing effect which causes the Faraday cage to appear electromagnetically much larger than it actually is, but without disturbing its electromagnetic conspicuousness which depends on the linear dimensions of the cage relative to a quarter wave length. For the sake of simplicity, this effect will be described only in relation to the E-field and E-flux of a passing electromagnetic wave, although an equivalent effect exists with regard to the I-I-field and H-flux.

FIG. 10 shows the side-contour of a motor car body 81 in silhouette projection. The wheel are omitted and, for purposes of simplicity the body 81 of the motor car is shown lying flat on, and electrically connected to, a perfect conductive earth surface 82. The bonnet, roof and boot areas of the car are identified as 83, 84 and 85, respectively.

On the extreme left and extreme right of FIG. 10 are shown a number of equally spaced horizontal lines 86-92, which represent the electrostatic field gradient of a vertically polarized wave, the quarter wave length 1 of which is large in comparison with the height 93 of the car body 81. As the equipotential lines 86-92 pass over body 81 they are deflected in an upward direction and theirmutual spacing is decreased. The cause of this deflection and compression effect is the electrostatic conspicuousness of body 81.

As previously explained, the Faraday cage does not participate in normal wave-propagation through the mutual inductance and mutual capacitance linkages of space. Instead the Faraday cage acts, at least in a first approximation, as a physically large, but still small relative to quarter wave length, three-dimensional shortcircuit of the E-field in space. A. conductive body of this kind may be referred to as an electrostatic clod or clump which severely disturbs normal E-field distribution in its vicinity as shown by the equipotential lines 86-92, as well as the electrostatic flux lines 101-120 which are at right angles withthe equipotential lines and are, therefore, also curved in the vicinity of the electrostatic clod 81.

It is the curvature of the electrostatic flux lines 101-120 towards the electrostatic clump 81, particularly the curvature of lines 106, 107, 113, 114 and 115 into the roof area 84 of the body 81 of the motor car which increases the interception of, or linkage with, the vertical electrostatic flux, thereby enlarging the apparent area of flux interception. The same effect occurs when viewingthe body 81 from the front or rear. The earth contour change effect is thus three-dimensional rather than two-dimensional.

The importance of the earth-contour-change effect in increasing the equalizing currents and their associated magnetic fields and fluxes within a Faraday cage can be assessed briefly by comparing interception ranges 121 and 122 in FIG. 10. Distance 121 represents the range of interception of the electrostatic flux by the roof 84 without curvature of the flux lines into the roof. Distance 122 shows that the range of interception with the flux line curvaturewhich actually occurs is nearly twice as large. Because the effect is three-dimensional, the result is an approximate quadrupling the roof-linkage with the E-flux. This causes the area of the electrostatic clump 81 to appear nearly four times larger electrically than it actually is. This substantial increase of the apparent intercepting area occurs without affecting the electromagnetic conspicuousness of the body 81 since there is no reduction of its physical or effective height relative to quarter wave length.

The same considerations apply to a Faraday cage which is predominantly magnetically linked with space. Thus the shorted single turn loop-type Faraday cage of FIG. 3 acts as a magnetic clod or clump if properly dimensioned with reference to a half-wave length. Such a magnetic clod intercepts a much larger magnetic flux than is indicated by its area alone, yet its electromagnetic conspicuousness, which is determined by its actual dimensions rather than by its apparent dimensional increase due to the earth contour change effect, is not affected.

The foregoing description of the performance of Faraday cage antenna systems using large natural" cages as primary antennas indicates that Faraday cage antenna systems are capable of drastically outperforming conventional antennas, provided steps are taken to achieve good total flux transfer efficiency. In addition to the above-described techniques for accomplishing this purpose, there are other means which can significantly improve the total flux transfer efficiency of a Faraday cage antenna system. For example, in the case of predominantly electrostatic field interception, the single post 173 connecting the ceiling 171 and floor 172 of the open Faraday cage antenna shown in FIG. 11 assures 100 percent current coupling efficiency provided the minute capacitive currents traversing the space inside the open cage are ignored.

The distance 174 between ceiling 171 and floor 172 is only a fraction of a quarter wave length, so that the open Faraday cage of FIG. 11 is functionally similar to a fully enclosed Faraday cage as indicated by the only slightly inwardly curved E-field rejection contour lines 175 and 176. Within the figure of rotation bounded by lines 175 and 176, ceiling 171 and floor 172, the E-field gradient is practically zero. Virtually all magnetic flux circulating around tubular center conductor 173 can be intercepted by the magneticcore 177 which can be either an air core or a ferrite. Magnetic core 177 partially or completely surrounds conductor 173 and carries a winding 178 having terminals 179 and 180.

The Faraday cage antenna of FIG 11 can be described as a shorted parallel plate condenser antenna. In spite of its low physical height compared to a conventional grounded unturned monopole, the Faraday cage antenna of FIG. 1 1 is highly efficient. The areas of ceiling 171 and floor 172 are large in comparison with the usual capacitive terminations of conventional monopoles or dipoles, and therefore link the antenna of FIG. 11 with a correspondingly larger crosssection of the vertical electrostatic flux, thus effectively replacing any lack of antenna height and providing a powerful coupling link with space for transmission or reception.

It is noted that the open Faraday cage antenna system of FIG. 11 .has all the properties of a good Faraday cage antenna system as set forth above, viz: (I) it is electromagnetically conspicuous, (II) it makes full use of the equalizing current between ceiling and floor thereby generating the largest possible magnetic flux for a given ceiling or floor area, and (III) although it is not fully enclosed, it has all necessary E-field equalizing properties of a fully enclosed Faraday cage. In addition, the Faraday cage antenna of FIG. 11 takes advantage of the previously mentioned energy conservation effect. As the E-field between the ceiling 171 and floor 172 is shorted out,-the H-field is strengthened, thereby increasing the equalizing current which flows through centerpost 173.

Electromagnetic conspicuousness, as defined above, manifests itself as an E-field and/or I-I-field disturbance by a three-dimensional conductive body inserted into the E-and/or H-tield. These electrostatic clods or clumps are electromagnetically conspicuous because they carry the necessary equalizing currents to short out or bridge the gradients of the E'fields and/or H- fields.

There is another form of electromagnetic conspicuousness applicable to this invention. This second form of electromagnetic conspicuousness which disturbs the original E- and H-field pattern does so not by shorting out or bridging the field gradients, but by interrupting them or separating the equipotential lines. This second form of electromagnetic conspicuousness can best be understood by comparing the open Faraday cage antenna system of FIG. 11 with the tuned antenna system shown in FIG. 12 which illustrates the second form of electromagnetic conspicuousness of a Faraday cage antenna.

The Faraday cage antenna of FIG. 12 is similar to a tuned parallel plate condenser antenna and includes an upper plate 191 and a lower plate 192, spaced apart by a distance 193 and connected by an inductor 194. The system is tuned by trimmer capacitor 195. The secondary antenna which is linked with the magnetic flux induced by the current flowing between the condenser plates is not shown. The secondary antenna may be of any of the air core or ferrite types previously described. It may be linked with condenser leads 196 and 197 or it may be secondary winding on inductor 194.

The coupling of the tuned antenna of FIG. 12 with space is almost entirely electrostatic since the radiation capacities of plates 191 and 192 dominate the small inductive linkage of inductor 194 and its leads 196 and 197 with space. In other words, the tuned parallel plate structure shown in FIG. 12 is a resonated rather than a resonant or self-resonant vertical dipole. The lack of length in terms of a quarter wave length of the intercepted wave is compensated by the heavy capacitive end loading of the antenna and the increased inductance of the antenna leads. Capacitive loading is provided by the radiation capacities of the two plates 191 and 192, and increased antenna lead inductance is provided by inductor 194.

The electromagnetic conspicuousness of the antenna shown in FIG. 12 when inserted into a homogeneous resonance which is absent in FIG. and present in FIG. 12. More Specifically, it is the resonance of the physically small, in terms of a quarter wave length antenna of FIG. 12 which develops at plates 191 and 192 potentials much larger than were present before the resonated antenna was inserted into the E-field. The small antenna of FIG. 12 thus has a large effective height and is able to reach further out into space as illustrated by the pattern of curvature of lines 201-206 towards it.

The shielding effect against the primary H-field is best explained by reference to equipotential magnetic field lines 207-211 shown in FIG. 12. Lines 207-211 are at right angles with the E-field lines 201-206 and the spacing between them increases as they approach the small tuned antenna thereby signifying a decrease of H-field intensity. Therate of increase of the spacing between lines 207-211 is, in accordance with the principle of conservation of electromagnetic energy, inversely proportional to the decrease of spacing between the E-field lines. Therefore, if the tuned parallel plate antenna of FIG. 12 is sufficiently small with reference to a quarter wavelength, it will keep the H-field out in spite of the fact that it is primarily a capacitive antenna and is wide open in the direction of polarization of the H-field.

The case of the inconspicuous antenna previously described falls between the case of the untuned antenna of FIG. 10 and the case of the tuned antenna of FIG. 12. The electromagnetically inconspicuous antenna is self-resonant grounded quarter wavelength monopole or the half wavelength floating dipole which is not capacitively. terminated and which does not disturb the natural E-field/H-field energy balance.

FIG. 13 shows another type of tuned open Faraday cageantenna using a highly efficient coupling transformer and secondary tuning. This antenna yields excellent performance in comparison with the conventional 3-foot whip indoor television antenna. The antenna of FIG. 13 includes an upper plate 221 and a v lower plate 222 and a large air core transformer 223 of rectangular cross-section which is located inside the open Faraday cage and fills a major portion of the cage. The reasons for the location and size of the transformer are: (l) The primary H-field of the passing electromagnetic wave is shielded from the space between the plates 221 and 222, thus avoiding undesirable magnetic coupling of the transformer windings 224 and 225 with the primary H-field, which, in the presence of random wave tilts would create unwanted directionality of the otherwise non-directional, E-field-linked antenna. (2) The larger the cross-section and length of the open air core transformer 223, the shorter is the required length for each of its windings 224 and 225, the lower are the surface current densities in these windings, and the higher the achievable Q. In addition, a high coupling coefficient between the windings can be achieved while minimizing unwanted stray capacities between the windings and between turns of the same winding. In other words, a large air core antenna transformer for matching a capacitive antenna to its tuning circuitry is to be preferred over a smaller transformer provided it can properly located in a magnetically shielded area. The spaceinside the open Faraday cage is ideally suited for this purpose. Naturally these considerations hold true also if the air core is replaced by a ferrite core.

As shown in FIG. 13, secondary winding 224 of transformer 223 is a broad, singleturn winding. Alternatively, secondary winding 224 may be a multipletum, spiral winding of the same width. As shown in FIG. 13, primary winding 225 is a four-turn, helical coil of flat tape connected to plates 221 and 222 at points 233 and 234, respectively. Alternatively, primary winding 225 may be conventional wire winding if a substantially larger number of turns is required. For purposes of simplicity, insulation between the windings is not shown. The combination of a helical winding 225 and a spiral or cylindrical winding 224 provides an excellent coupling between the two windings with minimum stray capacities which would be otherwise difficult to obtain, particularly if one of these windings must have very few or only a single turn (VHF and UHF region). Such coupling permits the open Faraday cage antenna to be tuned efficiently by its secondary circuitry. The secondary tuning element is variable inductor 226 which is connected between tab 227 of secondary winding 224 and grid 228 of input tube 229. The other terminal 230 of the secondary 224 is connected to ground 231 through a bias source 232. Certain constructional details of the assembly not pertinent to the invention have been omitted to avoid crowding the illustration.

While the transformer 223 in FIG. 13 is shown feeding a series resonant circuit, it may also shunt-feed a parallel resonant circuit. In either case, the effective inductance of the secondary winding 224 may represent any selected share of the total tuning inductance form a small fraction to percent (in which case inductor 226 would be omitted). Transformer 223 may also be a step-down or a step-up device.

It is also noted that the tuned parallel plate type open Faraday cage antenna of FIG. 13 may be operated as a monopole antenna by connecting plate 222 to earth, or by making it part of a large ground surface or counterpoise.

A particularly important application of the tuned parallel plate type open Faraday cage antenna according to the present invention is as a transmitting or receiving antenna for aircraft or space vehicles. FIG. 14 shows a typical example of the application of the present invention to such aerospace vehicles. More particularly, FIG. 14 shows a cross-section of an aircraft, missile, satellite or spacecraft 241, the outer metallic skin of which is divided into four. sections 242-245, which are insulated from each other at points 246-249. Two of these sections 242 and 244 act as plates of a tuned parallel plate-type open Faraday cage antenna. Plates 242 and 244 are curved in accordance with the surface contour of the aerospace vehicle 241. Viewed in the direction of vertical arrow 250 which represents the polarization axis of the E-field of signals which are to be transmitted or received, plates 242 and 244 may be circular, rectangular, or of any other chosen shape. Inductor 251 is connected between the plates242 and 244 so as to complete the primary circuit of the Faraday cage antenna system, whichmay then be tuned by means of parallel condenser 252, or by means of condenser 253 which is in parallel with secondary winding 254 of primary inductor 251. signals are injected into or taken from terminals 256 and 255 of secondary winding 254.

The aerodynamic advantages of the antenna system of FIG. 14 are obvious: The insulated antenna areas 242 and 244 are part of the outer skin of the vehicle but are not part of its main structural members such as beams or spars and thus the insertion of the insulating material 246-249 will not appreciably weaken the overall strength or rigidity of the vehicle. At the same time, since the antenna plates 242 and 244 are flush with the aerodynamically shaped surface of the vehicle the usual drag created by conventional antennas is avoided.

Electromagnetically, the antenna system of FIG. 14 has an unusual aspect. Because the Faraday cage antenna is a tuned dipole structure of less than half wavelength size, it is an electrostatically conspicuous body or clod which attracts the equipotential lines of the E-field in the same manner as the antenna of FIG. 12, thereby appearing taller and reaching out further into space. The remainder of the vehicle is an electrostatically conspicuous body or clod which repels the equipotential lines of the E-field, thereby appearing smaller in terms of the E-field gradient but larger in terms of E-flux interception.

The two different types of electrostatic clods have opposing effects upon the surrounding E-fleld and, in view of their close proximity, they mutually influence each other. The antenna system of FIG. 14 is, therefore, expected to be less efficient when incorporated into an aerospace vehicle than it would be if separate from the vehicle. Nevertheless, this reduced efficiency may compare favorably with conventional protruding antennas because such protruding antennas are often made undersized in an effort to compromise between antenna performance and the reduction of undesirable aerodynamic drag. On the other hand, the tuned Faraday cage antenna of FIG. 14, while handicapped by the presence of the main body of the vehicle, can comprise as large a portion of this body as desired in order to make up for the handicap (limited only by half wave dimensional considerations).

FIG. 15 shows an alternative Faraday cage antenna for aero-space vehicles which avoids the counteracting effect of the two different types of electrostatic clods upon each other. In FIG. 15 the Faraday cage antenna is an electrostatic clod of the same type as the aerospace vehicle itself. The Faraday cage antenna of FIG. 15 acts as the type of electrostatically conspicuous plates 242 and 244 of the antenna, which may create a discontinuity of electromagnetic conspicuousness in the vicinity of the antenna, the reduction of the efficiency of the untuned antenna of FIG. 15 is less than in the case of the tuned antenna of FIG. 14.

A further alternative design of a Faraday cage antenna system for aero-space vehicles which avoids the opposing effects of two different types of electromagnetic clods upon each other is illustrated in FIG. 16. The Faraday cage in FIG. 16 is a shorted, single turn loop and is therefore a magnetic rather than an electrostatic clod. The vehicle itself is also a magnetic clod and its influence upon the antenna is accordingly, minimal.

In FIG. 16 the insulated metallic ring 261 acts as the primary antenna of a Faraday cage antenna system similarly to the shorted, single turn, rectangular loop of FIG. 3. Ring 26lhas diameter 262 and can be an insulated cylindrical section of the skin or shell or an aircraft, missile, spacecraft or satellite 264. The length 263 of ring 261 can be selected in terms of a quarter wavelength of the intercepted radiation or multiples thereof to assure optimum electromagnetic conspicuousness. Ring 261 is electrically separated from body which collects E-flux but repels the equipotential I E-field lines. In general the antenna system of FIG. 15 is similar to that of FIG. 14, and the same parts are identified with the same numbers, but, in-FIG. 15, the shorting conductor 257 carries the E-field equalizing current between plates 242 and 244. The secondary ante'nna 258 is a single turn rectangular loop located near conductor 257. It will be understood however that any of the various forms of secondary antennas previously described may be employed.

As estimate of the efficiency of the antenna system in FIG. 15 can be made by comparing the performance of the antenna inside and outside of the body of the air or space vehicle. As previously explained in conjunction with FIG. 11, a shorted parallel plate type open Faraday cage collects the E-flux of a passing wave,

while shorting out or bridging the E-field. Except for incidental reflection and re-radiation effects-due to the fact that the body of the vehicle is somewhat'larger than the volume which is encompassed by the parallel the remaining skin or shell of vehicle 264 by suitable insulation material 265.

The secondary magnetic antenna is the two-turn loop 266 located on the inside of the primary antenna ring 261. Loop 266 has terminals 268 and 269 for connection to tuned or untuned input or output circuitry of a receiver or transmitter. It will be appreciated that the secondary antenna may have a large number of loops with or without a ferrite core or may be a complete toroidal winding inside the primary antenna ring 261.

The primary antenna ring 261, while insulated from the remaining body of the vehicle 264 by insulator 265 will unavoidably be coupled with various parasitic, shorted, secondary loops in the metal supporting structure such as the beams or spars of the vehicle. These parasitic loops donot detract appreciably from the ability of the primary loop, based on its cross-section, to collect magnetic flux from the passing electromagnetic wave. The parasitic loops do, however, rob the secondary antenna 266 of some of the secondary flux which would be available if the parasitic loops were not present. In spite of this loss of secondary flux, the Faraday cage antenna system shown in FIG. 16, provides an efficient coupling link for electromagnetic wave transmission and reception due to its large crosssectional and volumetric linkage with space. The Faraday cage antenna system of FIG. 16 does have directional properties which are generally undesirable because it is usually impractical to turn an aircraft, spacecraft or other vehicle in order to optimize wireless communication.

FIG. 17 shows a Faraday cage antenna system which provides for equivalent antenna rotation of the antenna and thus makes it unnecessary to turn the vehicle to optimize its communications.

The primary antenna of the Faraday cage antenna system of FIG. 17 is an adaption of the temple-type Faraday cage of FIG. 4 to an aircraft or space vehicle. The primary Faraday cage antenna includes two curved, insulated plates 271 and 272 which are flush with the aerodynamic contour of the vehicle itself. The four-comer posts 273-276 interconnect the ceiling plate 271 and the floor plate 272. Each corner post 273-276 carries a secondary antenna. More particularly, as shown in FIG. 17 the corner posts 273-276 carry four ferrite rings 277-280 having toroidal windings 281-284 which are series connected in diagonal fashion by means of leads 331 and 332 and are also connected through the two sets of terminals, 285/333 and 286/287 to a two-pole, two-position switch 288 having output terminals 289 and 290 for connection to a transmitter or receiver, not shown.

Equivalent antenna rotation is obtained by flipping the switch 288 to one side or the other. In one position secondary antenna coils 283 and 281 are connected to the output terminals 289 and 290 of the system. In this condition, posts 273 and 275 together with ceiling 271 and floor 272 of the primary Faraday cage antenna form a shorted loop which is angularly positioned for best reception of an incoming wave travelling at right angles to a line between posts 273 and 275. Assuming now that the direction of the incoming wave changes by 90 as the result, for example, of a right angle turn of an aircraft, switch 288 should be switched to the other position so as to connect coils 282 and 284 to output terminals 289 and 290. Switch 288, therefore, permits a selection of the best of two equivalent antenna positions so that for any orientation of the aircraft to the incoming wave, the loss of signal strength will not exceed 3 decibels provided the switch is turned to its most favorable position.

The four posts 273-276 and the secondary antennas 281-284 may have any desired, form such as, for example, that of simple transformers with air or ferrite cores and having their primary leads connected t0.the corners of the ceiling plate 271 and the floor plate 272 of the primary Faraday cage antenna. Further, the number of posts may be greater or less than four.

The switch 288 can be manual as shown in FIG. 17, but can also be automatic in order to relieve the operator of the burden of selecting the most favorable switch position. Any of several automatic switching techniques may be used, such as, for example:

1. Continuous switching back-and-forth at a rate exceeding the modulation frequency in order to avoid switching interference.

2. Mechanical linkage of the switch to the manual tuning control of the receiver or transmitter in such manner that more than one switchover occurs while the manual tuning control traverses the bandwidth of a single signal channel.

3. Relay operation of switch 288 wherein the operation of the relay coil is governed by a signal-seeking circuit such as are commonly used for automatic frequency control purposes in heterodyne receiversso that switch 288 will seek and come to rest on the stronger antenna-coil combination.

4. Twin receiver inputs including two identical but slightly differently trimmed RF tuning circuits in differential or parallel configuration, each RF tuning circuit being permanently connected to a different pair of secondary antenna coils, and both RF tuning circuits being mechanically linked to a single tuning control. The small difference in trimming of the two RF tuning circuits will cause the optimum amplitude and phase relation with the incoming signal to occur in slightly different positions to the tuning control for each of the two RF tuning circuits and, therefore, differential signals for delivery to the receiving system can always the found as the operator searches for the transmitting station regardless of the direction of the incoming wave. This is assured by the large phase shifts between the individual signal taken from the two antenna coil pairs, as their phases are independently shifted near resonance by the two, slightly differently trimmed, tuning circuits.

It will be appreciated that the principle of equivalent antenna rotation is not restricted to the use of two pairs of antenna coils as shown in FIG. 17. For example, single pair of coils on two adjacent posts may be preferred, if the length of interconnections between coil pairs offers problems at higher frequencies. In addition, it will be recognized that the switching can be accomplished by means of saturalble reactors, variable capacitors including varactor type diodes, etc. Furthermore, switching may be performed in the primary antenna leads rather than in the secondary leads as shown in FIG. 17.

A distinction has been made between two forms of electromagnetic conspicuousness of a Faraday cage antenna. More specifically the untuned Faraday cage antennas of FIGS. 10 and 11 deflect the equipotential lines of the E-field, while the tuned Faraday cage antenna of FIG. 12 attracts them. A similar distinction can be made with'regard to the disturbance of the H- field by Faraday cage antennas. As in the case of the disturbance of the E-field, whether or not the antenna is tuned determines the form of electromagnetic conspicuousness.

Functionally, the shorted parallel plate Faraday cage antenna of FIG. 11 has a magnetic counterpart in the shorted, single turn loop of FIG. 3. The former deflects the equipotential lines of the E-field, the latter deflects the equipotential lines of the I-I-field.

The functional counterpart of the tuned parallel plate Faraday cage antenna of FIG. 12 is shown in FIG. 18. The former attracts equipotential lines'of the E- field, and the latter attracts the equipotential lines of the I-l-field.

The open Faraday cage antenna of FIG. 18 includes single loop 291 which is tuned by capacitor 291 connected in parallel with the terminals 293 and 294 of the loop. The secondary antenna may be of any of the various types which have been described. In FIG. 18 the secondary antenna is a ferrite bar 295 which carries a coil 296 having terminals 297 and 298. This electromagnetically conspicuous antenna system attracts or concentrates the equipotential H-field lines, thereby appearing larger electromagnetically than it actually is, but without losing its inductive properties which are determined mainly by its circumference. To avoid inadvertent E-field interception, capacitor 292 and its leads should be placed inside the loop 291 which acts as an effective E-field shielding Faraday cage, rather than outside the loop 291as shown in FIG. 18 for convenience of illustration.

The secondary antenna 296 shown in FIG. 18 may, if desired, be replaced by a transformer, the primary of which is inserted into one of the leads 297 or 298 of tuning capacitor 292. Such a transformer may be in the form of a highly efficient large air core transformer such as that shown in FIG. 13. Further, tuning capaci- 

1. An antenna system comprising a passive primary antenna for intercepting and reradiating electromagnetic energy, said primary antenna comprising an electrically conductive vehicle body having a roof portion and a body portion and at least one electrically conductive member connecting said roof portion to said body portion, said vehicle body being of a size which disturbs the field patterns of an incident electromagnetic radiation so as to cause equalizing currents to flow within said Electrically conductive member; and a ferrite antenna for connection with a transmitter or receiver, said ferrite antenna being disposed in close proximity to said electrically conductive member, and having its axis skewed at substantially right angles to said electrically conductive member so as to couple at least a portion of the magnetic flux induced by said equalizing currents flowing in said electrically conductive member. 